A LOCOS process has been widely employed to form prior art device isolation structures. However, the prior art device isolation structures formed by the LOCOS process have several shortcomings such as, for example, the well-known problem of bird's-beak formations. Accordingly, various methods have been suggested to solve such problems in the prior art LOCOS process. One popular method is to form an STI structure as the device isolation structure. In brief, the conventional STI structure formation method comprises: forming a trench in a substrate by etching a field area to a depth of 3000 Å, filling a predetermined material into the trench by using high density plasma, and planarizing the surface of the resulting structure by a chemical mechanical polishing (hereinafter referred to as “CMP”) process.
FIGS. 1a through 1d are cross-sectional views illustrating a prior art process for forming an STI structure. Referring to FIG. 1a, a pad oxide layer 11 is grown on a silicon substrate 10. Subsequently, a pad nitride layer 12 is formed on the pad oxide layer 11. Both the pad oxide layer 11 and the pad nitride layer 12 function as buffer layers in the later etching process. A photoresist is then provided on the pad nitride layer 12 and patterned to expose the pad nitride layer 12.
Referring to FIG. 1b, the exposed part of the pad nitride layer 12, the pad oxide layer 11, and the silicon substrate 10 are then etched to a predetermined depth using the photoresist as an etching mask.
Referring to FIG. 1c, the trench is filled with an oxide layer 14 made of tetraethoxysilane (TEOS) which has a good step coverage characteristic.
Referring to FIG. 1d, a planarization process is performed to remove the pad nitride layer 12 and the pad oxide layer 11, thereby completing an STI structure 15.
However, in the prior art STI structure formation process, until a gate electrode is formed on the resulting structure, the formed STI structure is continuously exposed to etching environments. Thus, some parts of the STI structure are gradually and unintentionally etched by later etching processes. In particular, the oxide layer on the edges of the STI structure is easily etched due to its high activation energy. As a result, the top of the STI structure suffers from defects such as dents.